An electron moving at a speed of $ 5 \times 10^6\: ms^{-1}$ is shot through a sheet of paper which is $ 2.1 \times 10^{-4}\: cm$ thick. The electron emerges from the paper with speed of $ 2 \times 10^6\: ms^{-1}$ . The time taken by the electron in seconds to pass through the paper sheet is :